The invention generally relates to a liquid medium for producing a paste or slurry of powdered ceramic material from which fired ceramic articles or ceramic coated articles, having improved properties are produced.
More particularly, the invention relates to a method and a liquid medium for producing fired ceramics having improved density, color intensity, mechanical strength, and substrate to ceramic bond strength.
The method of the present invention is particularly suitable for the preparation of procelain pastes or slurries of the type employed by dental technicians in making dental appliances such as porcelain jacketed crowns, and the like.
It has been the standard practice for many years to utilize distilled water as a building medium, i.e., a transient binding agent, to enable laying-up or building-up of porcelain mix such as to a metal foundation of a dental appliance. While the use of water in forming a slurry of procelain powder has come to be a rather standard practice in the art, it is recognized that such practice is not without problems.
A significant problem is that shrinkage and distortion of a water slurry of porcelain powder, such as applied to a metal dental appliance foundation, often occurs upon firing. Additives have been tried in the wetting stage to develop green-biscuit, i.e. pre-fired, strength, and to minimize distortion and U.S. Pat. No. 3,973,970 is typical of such prior art attempts. However, the additive disclosed in U.S. Pat. No. 3,973,970 includes a component that is not transient which comprises a colloidal silica flux thereby providing a non-homogeneous fired porcelain.
Another problem is that as the porcelain slurry is applied to a metal foundation of a dental appliance, it tends to dry out whereby shrinkage or distortion occurs even before firing. U.S. Pat. No. 3,880,662 is typical of prior art directed to providing aqueous transient bonding agents that are intended to delay drying out by slowing down loss of water. While such attribute of the binding agent of U.S. Pat. No. 3,880,662 is not unlike one attribute of the practice of the present invention, i.e., keeping the porcelain mix wetter longer, such does not materially enhance the workability of the composition disclosed in the patent since the composition does not substantially eliminate the need to vibrate the porcelain mix as is generally the instance in the practice of the present invention.
Still another of the problems is that as the porcelain mix is applied to the metal foundation of a dental appliance, it is necessary to vibrate the mix on the surface being covered to insure that the porcelain particles move as closely together as possible to attempt to minimize shrinkage of the ceramic upon firing.